1. Field of the Invention
The present invention relates to powder-coating compositions, specifically to powder-coating compositions containing additives that improve the transfer efficiency of the powder-coating compositions. 2. Description of the Related Art
It is known that during the application of a powder-coating composition on to substrate, not all of the coating composition ends up on the substrate. The amount of powder-coating material being applied on to a substrate that actually ends up on the substrate as the powder coating relates to the transfer efficiency of the process. The transfer efficiency can be calculated by dividing the change in weight of the substrate by the change in weight of powder coating source dispensing the powder coating composition wherein the change in weight is the weight difference before and after the application of the powder coating. The transfer efficiency is conveniently expressed as a percent.
Although the powder coatings market is growing rapidly, one of the limiting factors for continued growth is the transfer efficiency. Typical powder coating application lines have first pass powder transfer efficiency from spray point to substrate of about 50-80%. The remaining powder has to be reclaimed and re-sprayed. Although this respray ability enables the powder coatings industry to cite lower coating costs and higher utilization of raw materials (of up to 95%) than liquid, solvent-based coatings it does decrease the efficiency of the process.
Furthermore, in some applications recycling powder is not possible owing to issues relating to appearance and quality of the finished part. One such area is in automotive clear coat applications. In these applications, the recycled material may contain lint or other foreign matter that is likely to cause defects in the finished coated automotive part.
Also, when using metallic or other dry blended systems (expensive color, etc.), the components may not transfer at equal rates (for example, metallic pigments or colors transfer at a different rate than the rest of the coating). Any reclaimed material looks different than the originally transferred coating. Thus spraying reclaimed material here amounts essentially to spraying to waste. This adds further cost to an already expensive dry blending process that is used to mix some of the components together to make the coating. Clearly there is a need for improving the transfer efficiency of powder coatings. The present invention fulfils such a need.
A transfer efficiency additive can positively affect the economics of the powder coating process. If recycling and reclaiming of powder-coating compositions are not feasible or desired, as mentioned above for automotive clear coats or when using metallic colors, then an efficient first-pass transfer becomes even more important. Two other factors that could be affected by a transfer efficiency additive are: 1) an improvement in the ability of the powder spray to penetrate into corners and close spaces, and 2) improving the ability of very small size powder coating particles to efficiently transfer and build on a metal substrate. Some industry sources estimate that an increase of about 3 to 5% in transfer efficiency would justify the cost involved in adding a transfer efficiency additive to achieve this increased transfer efficiency.
U.S. Pat. No. 5,073,579, Macholdt et al., Dec. 17, 1991, discloses a process for enhancing the electrostatic chargeability of powder coatings or powders intended for surface-coating solid objects, by homogeneously incorporating at least one triarylmethane, azine, thiazine and/or oxazine compound in an amount from about 0.01 to about 10 percent by weight into powder coatings or powders, the compounds mentioned being dissolved or dispersed in the powder coating system or powders.